Current sandwich structures used for aircraft flooring use only a single density honeycomb core in this construction. Heavy traffic on such flooring frequently causes failure or damage to the upper surface requiring that the flooring be replaced at great cost and inconvenience.
In order to increase the durability of such flooring panels and to increase their resistance to concentrated transverse compressive loads, either more skin material or higher honeycomb core densities or both must be used. Both of these remedies are basically unacceptable since they each substantially increase the panel weight. This is an unacceptable solution for aircraft structures where a minimum weight is always desired.
Many attempts to solve the problems stated above have been made. U.S. Pat. No. 4,937,125 describes one approach where a multilayer sandwich structure having a core interposed between and external and internal skin. The upper skin is a composite layer-polyester layer-honeycomb layer-metal sheet-glass fiber reinforced epoxy layer-composite layer. The core is a honeycomb. The internal skin is a metal sheet or a skin of the same composition as the external skin.
U.S. Pat. Nos. 4,336,292; 5,041,323 and 5,106,668 describe multi-layered panels comprised substantially of two or several subpanels--one possibly with a higher density core than the other(s)--bonded together to form a complete panel.
Accordingly, improving the upper surface properties of sandwich structures which are subject to in-use concentrated loads to make them more durable is highly desired. This will reduce life-cycle costs for airlines by decreasing the-necessity for replacing damaged panels. The instant invention provides that improved panel by tailoring the core to have enhanced properties just where needed (right below the upper skin) to achieve greater resistance to concentrated transverse compressive loads with a much smaller or even no weight penalty.
Honeycomb sandwich structures are widely used for aircraft flooring materials. In these structures, the in-plane and bending properties needed are those derived from design requirements of the airframe manufacturers. The out-of-plane properties are, by contrast, determined from test data on existing materials which are believed to meet the perceived requirements. It is known that the damage that causes panels to be replaced after use in aircraft occurs almost exclusively at the top surface of the panels and is due to concentrated out-of-plane loads. This damage is believed to be due to passenger foot traffic (most likely from high heels) administering localized concentrated loads on the panels.
The instant invention addresses the issue of floor panel durability by providing constructions which increase the resistance of aircraft flooring to concentrated out-of-plane loads without either increasing weight or significantly influencing cost when compared to standard uniform density honeycomb core construction. By using higher density materials near the top (loaded) surface of a sandwich structure, the resistance to damage such as core fracture, resin fracture, core buckling, etc. due to localized compressive loads is increased.
The use of foam-filled or foam cores in sandwich panels is well-known. The use of foam to fill honeycomb or lattice structures is a low density, low cost way to stabilize core structures. U.S. Pat. No. 3,249,659 demonstrates a method for introduction of foam into a honeycomb or lattice core at the top and bottom surfaces while leaving the middle section of the core untilled. The benefit of this method is a reduction of weight without a significant loss of core stabilization. Because the resins used to make-up the foaming adhesive of the U.S. Pat. No. 3,249,659 invention are untilled liquids and hence non-structural, no out-of-plane (transverse to the skin) mechanical property enhancement is provided. French 2,171,949 also describes a similar honeycomb product, useful for thermal and acoustic insulation purposes, wherein both the upper and lower surfaces are adhered to surface layers by use of an adhesive foam. The foam is sprayed onto the surface layer and said foam is not fiber-filled. Neither of these references relates to the instant products.
In order to avoid the use of epoxy resins which have poor flame, smoke and toxicity (FST) properties, U.S. Patent No. 4,135,019 teaches the use of carbon microballoons held in place by a bismaleimide resin. These structures do not enhance mechanical properties unless there is a significant increase in weight. Inspection of the composites made by the process of U.S. Pat. No. 4,135,019 as seen in Table 1 shows that the weight of such composites is 40% higher than the cited prior an. This is an unacceptable weight penalty.
U.S. Pat. No. 5,135,799 describes laminates used for insulation purposes which have a foam or honeycomb core attached to a fiber reinforced non-foaming cover layer of heat resistant material. U.S. Pat. No. 4,053,667 teaches fib-stiffened laminates obtained by molding and the use of stiffener beads. Foaming adhesives are not disclosed. U.S. Pat. No. 3,771,748 describes the use of foaming aahesives to stiffen the edge of a layer of honeycomb to bind it to support members. There is no disclosure that the foaming adhesive penetrates into the honeycomb core or that the foaming adhesive is fiber-filled.
In summary, none of the prior art focuses on the instant invention which is directed toward the solution to a practical problem for which there has been a long-felt need.